Adding unit tests

[NicolasGensollen]

I believe this would be a great addition and allow us to work in a more TDD fashion.
If we can decide on where these tests should be implemented and how they should be organized, I'll get started asap with tests for #587

I imagine two main different ways we could organize them:

• either we group them all in test/unittests/{engine,iotools,utils...}
• or we can put them in a testsfolder in each module (for example: Clinica/iotools/converters/adni_to_bids/tests/

Thoughts?

[ghisvail]

Since we have already adopted the convention of storing tests out-of-tree, I'd be in favour of keeping it for unit tests as well.

[omar-rifai]

Agreed. As discussed, we can mirror the pipelines hierarchy in a seperate test/unitest/ directory.